SDSS Image Analysis
Basics * Nanomaggies ** A "maggy" is the flux f of the source relative to the standard source f0 (which defines the zeropoint of the magnitude scale). Therefore, a "nanomaggy" is 10^{-9} times a maggy. ** The standard source for each SDSS band is close to but not exactly the AB source (3631 Jy), meaning that a nanomaggy is approximately 3.631\times10^{-6} Jy. However, our current understanding is that the absolute calibration of the SDSS system has some percent-level offsets relative to AB About SDSS Imaging Flags * See Robert Lupton's flags document * See the details of flags on SDSS DR7 page here * Also see the DR9 flags detail page here ** BINNED1: An object that is detected as greater than a 5 sigma peak (after smoothing with the local PSF) in a given band is flagged BINNED1 in that band. ** BLENDED, NODEBLEND, CHILD ** SATURATED, SATURATED_CENTER *** Note that a galaxy with a superposed saturated star in its disk, even if successfully deblended, will be flagged SATURATED, as some of the pixels in the object footprint are indeed saturated. ** EDGE, LOCAL_EDGE, DEBLENDED_AT_EDGE ** CR: means that the object contained a cosmic ray that was interpolated over. This does not mean that the object in question is a cosmic ray! ** PSF_FLUX_INTERP: This means that more than 20% of the PSF flux is interpolated over in the band in question, which may make one suspicious of the accuracy of the flux. ** CANONICAL_CENTER: If in some band, it is impossible to measure the center (due to being at the edge), one uses the center in another band, transformed according to the relative astrometry between the bands, and this flag is set. ** PEAKCENTER: If the centroiding algorithm can't find a better center, it will often simply report the position of the peak pixel in a given band. ** TOO_LARGE: Indicates an object which is still detected at the outermost point of the radial profile (a radius of over 4 arcmin), or at least one child in a deblend is larger than 1/2 a frame. This is indicative either of a very large object, or a poorly determined sky, or a particularly horrific deblend. ** ELLIPFAINT: Objects whose isophotal shape could not be measured. ** PETROFAINT: the Petrosian radius is measured at a very low surface brightness level ** NOPETRO: the code was unable to measure the Petrosian radius. ** BAD_RADIAL: It is often true that the last bin in the radial profile of an object goes slightly negative. When this happens, the BAD_RADIAL flag is set; it can usually be ignored. Skills Getting Images and Necessary Links * [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fGetUrlExpEq&t=F fGetUrlExpEq]: Returns the URL for an ra,dec at the Explorer tool. ** select dbo.fGetUrlExpEq(@ra,@dec) ** The format for this URL is: http://localhost/en/tools/explore/obj.asp?id=2255029915222048 * [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fGetUrlFitsCFrame&t=F fGetURLFitsCFrame]: Get the URL to the FITS file of a corrected frame given the fieldID and band ** select dbo.fGetUrlFitsCFrame(@fieldid,'@band') Reference * Information in the Project Book (PBOOK)-[http://www.astro.princeton.edu/PBOOK/datasys/datasys.htm Datasystems Section] could be useful * Improved Background Subtraction for the Sloan Digital Sky Survey Images ** Blanton, Kazin, Muna, Weaver & Price-Whelan 2011, AJ * An Improved Photometric Calibration of the Sloan Digital Sky Survey Imaging Data ** Padmanabhan et al. 2008, ApJ * Sloan Digital Sky Survey Standard Star Catalog for Stripe 82: The Dawn of Industrial 1% Optical Photometry ** Ivezic et al. 2007, AJ * Systematic trends in Sloan Digital Sky Survey photometric data ** Bramich & Freudling 2012, MNRAS Useful Information in the Database Useful Information from Photometric Catalogs: ** Index: objid, run, rerun, camcol, field, obj, mjd ** Position: ra, dec, raErr, decErr, b, l ** mode: 1: primary, 2: secondary, 3: other *** Useful function: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoMode&t=F fPhotoMode], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoModeN&t=F fPhotoModeN] ** type: Type classification of the object *** Including UNKNOWN=0; COSMIC_RAY=1; DEFECT=2; GALAXY=3; GHOST=4; KNOWNOBJ=5; STAR=6; SKY=8 *** Useful functions: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoType&t=F fPhotoType], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoTypeN&t=F fPhotoTypeN] ** clean: Clean photometry flag (1=clean, 0=unclean). ** probPSF, probPSF_u,g,r,i,z: Probability that the object is a star. ** insideMask: Flag to indicate whether object is inside a mask and why *** Including INMASK_NOT_IN_MASK; INMASK_BLEEDING; INMASK_BRIGHT_STAR; INMASK_TRAIL; INMASK_HOLE; INMASK_POOR_SEEING; INMASK_BAD_SEEING *** Useful function: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fInsideMask&t=F fInsideMask], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fInsideMaskN&t=F fInsideMaskN] ** flags, flags_u,g,r,i,z: Photo Object Attribute Flags *** Pay Attention to the Following ones: BRIGHT, EDGE, BLENDED, CHILD, NODEBLEND, NOPETRO, DEBLEND_TOO_MANY_PEAKS, COSMIC_RAY, SATURATED, SUBTRACTED, TOO_LARGE, SATUR_CENTER, INTERP_CENTER, PSF_FLUX_INTERP *** Useful functions: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoFlags&t=F fPhotoFlags], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fPhotoFlagsN&t=F fPhotoFlagsN] ** rowc, colc, rowcErr, colcErr, rowc_u,g,r,i,z, colc_u,g,r,i,z ** psfMag_u,g,r,i,z; psfMagErr_u,g,r,i,z, in mag ** psfFlux_u,g,r,i,z in nanomaggies; and psfFluxIvar_u,g,r,i,z in nanomaggies^{-2} ** aperFlux7_u,g,r,i,z; aperFlux7Ivar_u,g,r,i,z: Aperture flux and Inverse variance of aperture flux within 7.3 arcsec ** extinction_u,g,r,i,z: Extinction in each band ** psffwhm_u,g,r,i,z: FWHM in each band ** airmass_u,g,r,i,z: Airmass at time of observation in each band ** fieldID: Link to the field this object is in ** thingID, nObserve, nDetect, nEdge ** score: Quality of field (0-1) ** calibStatus_u,g,r,i,z: Calibration status *** Including: PHOTOMETRIC, UNPHOT_OVERLAP, UNPHOT_EXTRAP_CLEAR, UNPHOT_EXTRAP_CLOUDY, UNPHOT_DISJOINT, INCREMENT_CALIB *** Useful functions: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fCalibStatus&t=F fCalibStatus], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fCalibStatusN&t=F fCalibStatusN] ** nMgyPerCount_u,g,r,i,z: in nMgy/count ** TAI_u,g,r,i,z: time of observation (TAI) in each filter Useful Information from [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=Field&t=U Field] catalog: ** Index: fieldID, run, rerun, camcol, field ** Number of Objects: nTotal, nObjects, nChild, nGalaxy, nStars ** Number of Bright Objects: nBrightObj_u,g,r,i,z ** quality: Quality of the field *** Including BAD, ACCEPTABLE, GOOD, MISSING, HOLE *** Useful function: [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fFieldQuality&t=F fFieldQuality], [http://skyserver.sdss3.org/public/en/help/browser/description.asp?n=fFieldQualityN&t=F fFieldQualityN] ** Astrometry terms: a,b,c,d,e,f in _u,g,r,i,z ** Center of the field: ra, dec ** Min and Max Ra/Dec: raMin, raMax, decMin, decMax ** Pixel scalte: pixScale in arcsec/pix ** Saturation level: saturationLevel_u,g,r,i,z in counts ** Effective area of PSF: nEffPsf_u,g,r,i,z in pix ** Sky Estimate from Postage Stamp Pipeline: skyPsp_u,g,r,i,z in nmgy/arcsec^2 ** Frames sky value: skyFrames_u,g,r,i,z in nmgy/arcsec^2 ** Sigma of pixel values: sigPix_u,g,r,i,z in nmgy/arcsec^2 ** Fractional Sigma of Sky Value Distribution: skySig_u,g,r,i,z in mag *** Noise_{Sky}=skySig\times sky\times ln(10)/2.5 ** Error in PSP estimate of sky: skyErr_u,g,r,i,z, in nmgy/arcsec^2 ** Rate of change in sky value along columns: skySlope_u,g,r,i,z, in nmgy/arcsec^2/field ** Number of bins in PSF profile (in fieldProfile table): nProf_u,g,r,i,z ** Number of stars used in PSF measurement: psfNStar_u,g,r,i,z ** Photometric uncertainty due to imperfect PSF model: psfApCorrectionErr_u,g,r,i,z, in mag ** Inner gaussian sigma for composite fit: psfSigma1_u,g,r,i,z ** Outer gaussian sigma for composite fit: psfSigma2_u,g,r,i,z ** Ratio of inner to outer components at origin: psfB_u,g,r,i,z ** Value of power law at origin in composite fit: psfP0_u,g,r,i,z ** Slope of power law in composite fit: psfBeta_u,g,r,i,z ** Width of power law in composite fit: psfSigmaP_u,g,r,i,z ** Effective PSF width from 2-Gaussian fit: psfWidth_u,g,r,i,z, in arcsec ** Sigma of inner gaussian in 2-Gaussian fit: psf2GSigma1_u,g,r,i,z, in arcsec ** Sigma of outer gaussian in 2-Gaussian fit: psf2GSigma2_u,g,r,i,z, in arcsec ** Ratio of inner to outer components at origin: osf2GB_u,g,r,i,z ** Gain averaged over amplifiers: gain_u,g,r,i,z ** Dark variance: darkVariance_u,g,r,i,z ** Quality of field: score, from 0 to 1 ** nanomaggies per count due to instrument: aterm_u,g,r,i,z, in nmgy/count ** atmospheric k-term at reference time in calibration: kterm_u,g,r,i,z ** Calibration status bitmask: calibStatus_u,g,r,i,z ** Image status bitmask: imageStatus_u,g,r,i,z ** nanomaggies per count: nMgyPerCount_u,g,r,i,z, in nmgy/count ** Start and End of Field in Stripe coords, muStart, muEnd, nuStart, nuEnd, in arcsec